Tuesday, January 5, 2016

Mountains: How are they formed?

When beholding the sheer size and majesty of mountains, ancient humans could not help but feel that they were standing in the presence of something… godlike. And within the belief systems of many ancient cultures, it was generally felt that mountains were something spiritual – either serving as the home of the Gods, a result of their activity, or a place to get closer to God.

Thanks to modern geology, we now know the true story of how mountains are formed. Simply put, they are the result of tectonic forces or volcanism. But knowing this has not diminished their impressive and awe-inspiring nature. When a geological formation is created through forces that can only be described as titanic, this is to be expected. But just how are mountains formed?
In truth, there are three ways in which mountains are formed, which correspond to the types of mountains in question. These are known as volcanic, fold and block mountains. All of these are the result of plate tectonics, where compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features.

Over the course of many million years, these uplifted sections are eroded by the elements – wind, rain, ice and gravity. These gradually wear the surface of the mountains down, cause the surface to be younger than the rocks that form them, and lead to the types of formations and distributions we are familiar with today.

The East side of the Matterhorn, a fold mountain that measures 4,478 meters in height, mirrored in lake Riffelsee. Credit: Wikipedia Commons/Dirk Beyer

Volcanic Mountains:

Volcanic mountains are formed when a tectonic plate is pushed beneath another (or above a mid-ocean ridge or hotspot) where magma is forced to the surface. When the magma reaches the surface, it often builds a volcanic mountain, such as s shield volcano or a stratovolcano. Examples of this sort of mountains include Mount Fuji in Japan, Mauna Kea in Hawaii, Nyamuragira in the Democratic Republic of Congo, Skjaldbreiður in Iceland and Mount Etna in Sicily.

At other times, the rising magma solidifies below the surface and forms dome mountains, where material is pushed up from the force of the build-up beneath it. Examples of this formation include Navajo Mountain in San Juan County, Utah; the Chaitén lava dome of Chile, Torfajökull in Iceland, and Mount St. Helens in Washington State.

Fold Mountains:

As the name suggests, fold mountains occur when two tectonic plates collide at a convergent plate boundary, causing the crust to overthicken. This process forces the less dense crust to float on top of the denser mantle rocks – with material being forced upwards to form hills, plateaus or mountains – while a greater volume of material is forced downward into the mantle.

The Jura Mountains, a series of sub-parallel mountain ridges located in the Alps, are an example of fold mountains. Other examples include the “Simply Folded Belt” of the Zagros mountains, which extends from northern Syria and southern Turkey to eastern Iran and the Persian Gulf. There is also the Akwapim-Togo ranges in Ghana and the Ridge-and-Valley Appalachians in the Eastern United States.

But perhaps most famous is the Himalayan mountain chain, located between northern India and Nepal. This chain formed as a result of the collision between the Indian subcontinent and Asia some 25 million years ago, and has given rise to the tallest mountain in the world – Mt. Everest.Block Mountains:

Block mountains are caused by faults in the crust, a seam where rocks can move past each other. Also known as rifting, this process occurs when rocks on one side of a fault rise relative to the other. The uplifted blocks become block mountains (also known as horsts) while the intervening dropped blocks are known as graben (i.e. depressed regions).

Examples of this type of terrain can be found in the Upper Rhine valley, the Vosges mountains in France, the Black Forest in Germany, and the Vindhya and Satpura horsts in India. There is also the East African Rift, an active continental rift zone with several active volcanoes that extends from Eritrea to Mozambique.

Satellite image of the East African Rift, taken on December 18th, 2002. Credit: NASA/GSFC/METI/Japan Space Systems/U.S.-Japan ASTER Science Team

Mountain Erosion:

As noted, the final way in which mountains are formed is through erosion. This occurs during and after an uplift, where a newly formed mountainous region is subjected to the effects of wind, water, ice, and gravity. These forces actively shape the surface of mountain ranges, wearing down the exposed surfaces, depositing sediment in alluvial flows, and leading to the formation of characteristic landforms.

These include pyramidal peaks, knife-edge arêtes, and bowl-shaped cirques that can contain lakes. Plateau mountains, such as the Catskills, are formed from the erosion of an uplifted plateau. And after millions of years of erosion, mountains may cease to exist entirely.

Given the size and scale of a mountain, the immense forces involved in their creation, and the immense amount of time it takes to shape and form them, it is little wonder why they are considered such a big deal. Between their religious significance (i.e. Mount Zion, Mount Olympus, Mount Ararat, and Mauna Kea, to name a few), their scenic value, the challenge they present, and their importance to the Earth sciences, these geological formations continue to enjoy a special place in our hearts, minds and culture.

As we explore other planets, we have also found new and impressive mountain formations that have taught us much about the geological activity and composition of other worlds. For example, there the volcanic mountain on Mars known as Olympus Mons, which just happens to be the largest mountain in the Solar System. And this is merely a drop in the bucket. Wherever there’s a geologically active planet, there’s mountains to be found!

We have written many articles about mountains here at Universe Today. Here’s one on Fault-Block Mountains, one on Volcanic Mountains, one on Fold Mountains, and one on Dome Mountains. And here’s an article about The Clearest Skies On Earth.